The present invention relates to isothermal forging and isothermal sizing of titanium containing workpieces. Isothermal shaping of metal contemplates isothermal forging where substantial amounts of new surface are generated, or isothermal sizing wherein a previously contoured workpiece is brought within predetermined tolerances and wherein the die and the workpiece are heated and maintained at a predetermined temperature during the shaping operation. The dies are made of the so-called superalloy materials which contain substantial amounts of nickel and chromium.
Hot shaping of metal is not new. An important work in this field is the patent to Dolch U.S. Pat. No. 3,154,849 which describes a process including the precoat lubrication of the interface between the die and the metal workpiece with a vitreous composition characterized by the presence therein of silica and lead oxide. The Dolch disclosure relates to impact forging. The lubricant there disclosed is applied as a slurry by spray gun application to the workpiece. An organic precoat medium composed of a solvent and/or a diluent and a resinous vehicle was used to assist application of the lubricant to the workpiece. As the temperature of the workpiece was raised to forging temperature, the organic solvent, e.g., alcohol evaporates and the resinous portion which serves as a temporary binder is ultimately thermally decomposed as the temperature is further increased.
In isothermal forging and sizing, both the die and the workpiece are elevated to the forging or sizing temperature, and rather than impact shaping, a slow steady high pressure is applied by hydraulic means. Isothermal sizing as opposed to isothermal forging refers to a relatively light reduction taken in the workpiece to bring a forged workpiece to final net dimensions and surface finish. Ease of release or separation from the die is vital and accumulation of material from the lubricant or separation compound is not tolerable for an isothermal forging or sizing operation.
Initial forging lubricants in this field were developed as a result of prior experience in incrementally increasing die temperature in conventional forging processes to reduce chilling in workpieces and were composed of graphites suspended in water. It was later discovered that sodium silicate provided a suitable vehicle for graphite, and compositions so produced worked quite well at the higher conventional die temperatures.
As component precision requirements exceeded the capabilities of conventional forging, regardless of die temperature, isothermal processing studies were initiated. In isothermal processing on dies at 1350.degree.-1750.degree. F., graphite even with minor amounts of sodium silicate was found to be ineffective because the die loading had to be so high for substantial metal movement that damage to the die itself was encountered. Also because of the very high die temperatures (1350.degree.-1750.degree. F.) spraying of the lubricant on the dies had to be abandoned in favor of introducing the lubricant on the workpiece as a precoat. It was found subsequently that by increasing the vitreous or glass component in the precoat lubricant, die life was improved and greater metal movement could be achieved. Increasing the glass component in these systems appeared satisfactory up to about 50% glass content. At higher concentrations of glass with a solid lubricant dispersed therein, there was loss in surface integrity which necessitated a machining operation to produce the proper surface on the articles. Glass build up in the dies and component removal from the dies were also problems with high concentrations of vitreous material, i.e., greater than 50%.
Various other lubricant compositions have been tried, some with considerable success such as shown in Ser. No. 653,382 filed Jan. 29, 1976, now U.S. Pat. No. 4,096,076 to Spiegelberg. This composition depends upon boron nitride in an amount less than 50% by weight as a solid lubricant in a boron trioxide containing vitreous phase most applicable to large "near-net" titanium components that are later machined all over.
In summary, the prior art in providing a lubricating composition for hot forging techniques has proceeded with the concept of a minor amount of a relatively soft dry lubricant, e.g., graphite and/or boron nitride, suspended in a fused glasslike vehicle. Problems have been encountered in isothermal hot forging techniques with effectiveness of the lubricant, pressure required to move considerable amounts of metal, build up of lubricant in the die, poor surface characteristics of the finished piece, etc. Moreover, prior art compositions have been found to have a narrow thermal spectrum, e.g., about 150.degree. F., over which they are useful.
The present invention is concerned with improved glass-graphite compositions utilizing in equal or major amount of solid lubricant for use an isothermal forging or sizing operations. These improved coating compositions demonstrate, for example, with titanium or titanium alloy workpieces, desirable properties in the hot forging or sizing thereof. The high concentration of graphite exerts a self cleaning effect on the dies and greatly alleviates the problem of glass build-up in the dies. The workpiece separates better from the dies and is substantially free of "orange peel" or "egg shell" or other surface texture blemishes. Limiting of the particle size of the glass component appears to be responsible for the improved performance even though the glass is a liquid vehicle for the graphite under forging or sizing conditions. These compositions also have a favorable influence on the die loading because they reduce the force required to effect shaping. This results, in turn, in improved die life.
It has been found that reduction of the particle size of the glass component results in the foregoing improvements, and in particular has a critical influence on the surface characteristics of the finished workpiece. Isothermal sizing and/or forging procedures utilizing a graphite-glass lubricant composition where the graphite is present by weight in an amount equal to or greater than the amount of vitreous component and a commercially available vitreous component having a particle size of approximately 60 mesh has been tried. Lubricants containing this glass component resulted in finished pieces which were characterized by surface blemishes rendering them commercially unsuitable. When the isothermal forging or sizing procedure was improved to utilize lubricants containing a glass component wherein the particle size thereof had been reduced below 200 mesh, commercially satisfactory products were produced.
Reference may be had to the patent to Watmough et al. U.S. Pat. No. 3,635,068 for a disclosure of hot forging of titanium and titanium alloy workpieces utilizing glass and glass-graphite lubricant compositions.